In conventional vehicle drives having only an internal combustion engine, for regulating the rotational speed, device controls are used which, after the internal combustion engine is started, regulate its rotational speed based on a difference between a setpoint rotational speed and an actual rotational speed, as described in the document DE 600 15 820 T2, for example. However, such rotational speed regulation is not suitable for compensating for rotational speed oscillations which occur due to rotational speeds that change in rapid succession when an internal combustion engine is engaged with a hybrid drive.
FIG. 1 shows an example of a drive train of a hybrid vehicle having an internal combustion engine 101 which is coupleable to an electric motor with the aid of a separating clutch 103. The drive train also includes a torque converter 107 situated downstream from electric motor 105, and distributor gearing 109. For damping rotary vibration, separating clutch 103 usually includes one or multiple bow springs which are compressed when separating clutch 103 rapidly engages for starting internal combustion engine 101. This causes excitation of rotary vibration between, internal combustion engine 101 and electric motor 105, which may adversely affect the rotational speed regulation due to strong torque deflections of a rotational speed regulator. In addition, these oscillations have a frequency which, due to the drive inertia, among other factors, is too high to allow efficient compensation for the rotary vibrations, so that the above-mentioned torque deflections of the rotational speed regulator may have a particularly intense effect.
FIG. 2 shows an example of a curve of the rotational speed oscillations which arise when separating clutch 103 is engaged at point in time 201. Curve 203 illustrates a rotational speed curve of the internal combustion engine, while curve 205 shows a rotational speed of the electric motor. In FIG. 2 it is clearly apparent that the rotational speed of electric motor 105 as well as the rotational speed of internal combustion engine 101 oscillate after separating clutch 103 engages. However, it is not possible to efficiently compensate for these oscillations on the basis of a single fixed setpoint value.